A rare deep-blue diamond ring worth more than £1 million could make history when it goes on sale this month.

The "fancy" diamond weighs 5.30 carats and is set in a Trombino ring made by renowned Italian jeweller Bulgari, who is favoured by Hollywood film stars. The term fancy is used to describe a diamond of intense colour and the colour fancy deep-blue is one of the rarest in the world.

The cushion-shaped diamond is set horizontally within a mount of brilliant-cut diamonds and baguette-cut diamonds.

Blue diamonds are structurally very pure and account for less than 1% of all diamonds mined.

The ring is thought to have been made in 1965 and is estimated to fetch up to £1.5 million at Bonhams Fine Jewellery sale on April 24.

The world record price for a fancy deep-blue diamond is 1,038,000 US dollars (about £686,000) per carat but, according to staff at Bonhams, the Bulgari ring has the potential to break this.

Jean Ghika, director of the jewellery department, said: "There have only been 30 diamonds of this size and colour that have come up for sale globally over the last 10 years which is a fraction of the percentage of white diamonds that have been sold during the same period of time.

"There are a number of people who are interested in this kind of stone because they don't come on the market very often. It attracts connoisseurs, collectors and people who just simply want to wear it because it's a wonderful stone."

In September 2011 Bonhams sold a fancy vivid-blue diamond ring by Bulgari for £1.9 million.

The ring, set with two pear-shaped diamonds, more than doubled the pre-sale estimate which is why experts predict the Trombino Bulgari ring will produce another record-breaking sale.